The present invention relates to the conversion of a signal such as a video signal to a signal of a desired frequency characteristic, and more particularly to a signal processing method and apparatus suitable for improving the S/N ratio and waveform distortion of a signal in a transmission system.
A method of frequency-modulating (FM) a video signal and transmitting (or recording and reproducing) the resulting video signal is generally used in a recording and reproducing apparatus such as a video tape recorder or a video disc player which records and reproduces a video signal, or in a signal transmission medium such as a satellite broadcast. In order to prevent the reduction of the S/N ratio of a signal received in such an FM transmission system, a signal processing method is conventionally used in which a high-frequency region component of the modulated signal is beforehand emphasized, namely, preemphasized, and a high-frequency region component is suppressed or deemphasized after the demodulation of the FM signal.
In order to transmit a signal with high fidelity in such signal processing method, the following equation must be satisfied irrespective of frequency, EQU G.sub.1 (S).times.G.sub.2 (s)=k (1)
where G.sub.1 (s) is the transfer function of the preemphasis circuit which emphasizes the high-frequency region component of the signal, G.sub.2 (s) is the transfer function of a deemphasis circuit which suppresses the high-frequency region component of the signal conversely, s=j.omega. where is the angular frequency of the signal, and k is a constant.
Unless equation (1) is satisfied, a phase distortion and an amplitude distortion would be produced in the transmitted (recorded and reproduced) signal, and the reproduced signal would be distorted. A preemphasis circuit and a deemphasis circuit which satisfy the equation (1) and which have the following transfer functions can easily be economically realized using resistors and capacitors, so that they are used very often conventionally, ##EQU1## where EQU T.sub.1 &gt;T.sub.2
In this conventional method, however, the phase characteristics of the preemphasis and deemphasis circuits are not considered.
A method of improving the phase characteristic of the emphasis circuit is well known, as disclosed in Japanese Laid-Open Publications JP-A Nos. 53-131814 and 53-131815 and Japanese Patent Publication JP-B No. 61-8632. These references do not provide a deemphasis method enough to satisfy the equation (1).
A method of improving the S/N ratio of a signal using an emphasis circuit represented by the equation (2) is well known, as disclosed in Japanese Laid-Open Publication JP-A Nos. 59-221126 and 60-7279. Any of these references does not provide means for operation of the deemphasis circuit in a stabilized manner.
The phase characteristic of the emphasis circuit does not provide good linearity according to the conventional methods, as will be obvious from the equation (2), so that if, for example, a square pulse signal is preemphasized, as mentioned above, an overshoot or an undershoot of large amplitude would occur only in the direction of rise or fall of the signal. If frequency modulation is performed using this signal as a modulating signal, a quantity of frequency deviation increases correspondingly to thereby increase the zone which the FM signal occupies to require a wider transmission band. The zone of a signal which can be recorded in a medium is limited by itself in a recording and reproducing device such as a video tape recorder or a video disc player. In the conventional preemphasis system, a large-peak waveform would be produced in one direction for a high-frequency region component of the signal, so that the instantaneous frequency of the FM signal would be extremely high for the overshoot, a signal of high frequencies cannot be sufficiently reproduced due to the band restriction of the medium to thereby produce a so-called inversion (the generation of noise with a black horizontal line in the configuration area in which the video signal changes from black to white). The instantaneous frequency of the FM signal would be extremely lowered for an undershoot to thereby generate beat-like noise in the image configuration area due to a so-called fold of a spectrum to thereby deteriorate the reproduced image quality greatly. In order to prevent this phenomenon, the overshoot and undershoot waveforms of the signal after emphasis are generally clipped (amplitude-limited). By the waveform clipping, part of the signal would be lost, the equation (1) would no longer hold and the reproduced waveform would greatly be distorted. In order to prevent this situation, a method is generally used in which a quality of emphasis is reduced or a quantity of frequency deviation is reduced. Although the waveform distortion is improved, an essential problem would still remain in which the S/N ratio would be deteriorated, of course.